Radiothermoluminescence dosimeters and materials therefor

ABSTRACT

Complex oxide luminescent material consisting of magnesium oxide-silicon dioxide and containing trace amount of terbium or cerium as an activator shows strong thermoluminescence with glow peak at ca. 190*C under excitation by means of electron beam or various radiations and thus is useful as the phosphor for thermoluminescent dosimeter.

[ Oct. 30, 1973 RADIOTHERMOLUMINE SCENCE DOSIMETERS AND MATERIALSTHEREFOR [75] Inventors: Noboru Kotera, Kamakura; Satoru Nishikawa,Yokosuka; llitoshi Sakamoto, Chigasaki, all of Japan [73] Assignee: DaiNippon Toryo Kabushiki Kalsha, Osaka-shi, Japan [22] Filed: Apr. 27,1972 [21] Appl. No.: 248,251

Related U.S. Application Data [62] Division of Ser. No. 62,1l6, Aug. 7,I970.

2,577,161 12 1951 Smith 252 301.4 F 3,468,810 9 1969 Mizuno.... 252/3014F 3,173,850 3/1965 1166a 250 83 CD 2,800,589 7/1957 Levy 250/83 c1)3,282,855 11/1966 Palmer et a]. 250 83 c1) Primary Examiner-Harold A.Dixon Attorney-Richard C. Sughrue [57] ABSTRACT Complex oxideluminescent material consisting of 5 us C 250 459 250/71 R, 252/3014 Fvmagnesium oxide-silicon dioxide and containing trace 250/473,250/484amount of terbium or cerium as an activator shows 51 1111.01. G0lt 0/2Strong thermoluminescence with glow peak at ca. [58] Field of Search250/71 R 83 CD- 190C under excitatim by means electm" beam 25273014various radiations and thus is useful as the phosphor forthermoluminescent dosimeter.

[56] References Cited UNITED STATES PATENTS 10 Claims, 6 Drawing Figures3,671,452 6/1972 ,Inoue et al. 252/301.4 F

m ooesio z rb 1.1.1 2 O (I) LU 2 i l- D 0') l 2 l O 11.1 E F- m 2 Lu E EO L l i 1 -5 4 5 "2 l0 lo 10 IO :0

y Tb g-otom mol) PATENIEUnmsoms 3,769,510

I swam 20F 4' THERMOLUMINESCENCE INTENSITY 0 I00 200 30o 40o TEMPERATURE(c) FIG.3

PATENTEUHCI 30 I975 SHEET 3 BF 4 EXPOSURE DOSE X RAY) EX POSURE DOSE T-RAY) FIG.4

PATENTEDOUBO ma 3.7691510 SHEEY u [If 4 RADIATION THERMOLUMINESCENCE (IOm HEATING EXPOSURE STORAGE" READ OUT [GLOW CURVE *1 (D Z w l- Soc DOSE ETEMP. (C)

FIG.5

RADIOTHERMOLUMINESCENCE DOSIMETERS AND MATERIALS THEREFOR This is aDivision of application Ser. No. 62,1 16, filed Aug. 7, 1970.

This invention relates to radiothermoluminescence dosimeters adapted foruse in detection and measurement of exposure dose of irradiatedradiation.

The utilization of X-ray emitted from radioactive materials such as 60electron beam obtained from electron accelerating apparatus, X-rayobtained from X-ray generating apparatus, etc. has recently beenincreasing in industrial and medical fields. X-ray, for example, isutilized in diagnosis in medical purposes and nondestructive testings inindustrial fields, whereas 60 is not only used in medical fields butalso utilized for exposure of radiation for improvement of species inagricultural fields or for preservation of food in fresh state and alsofor synthesis and improvement of industrial materials, and suchutilization is presumed to be still increasing hereafter. In order toestablish precautions against radiation, therefore, it is indispensableto measure the exposure dose of radiation by means of a simple procedureand various dosimetry methods have been developed for this purpose.

Particularly the thermoluminescent dosimeter utilizing thethermoluminescent phenomenon of phosphor has attracted the attention inthe fields of health physics, radiology, etc. and has been employedwidely in these fields because of the advantages thereof such as simpleoperation, compactness, availability in various forms such as powder,tablet, etc., and ability for precise measurement of cumulative doseover a wide range of various radiations. I

Radiothermoluminescent materials are provided with an ability toaccumulate the energy absorbed therein when said materials are exposedto radiations such as X-ray, and to emit said accumulated energy asluminescence, i.e. thermoluminescence, when said materials acquirethermal energy for example by heating. COnsequently the quantitativedetermination of exposure dose can be realized by measuring the lightsum or light intensity of said thermoluminescence.-

Although the mechanism of thermoluminescence is specific to eachphosphor, said mechanism can be qualitatively explained as follows: Inradiothermoluminescent materials, impurity elements of crystallinelattice defects present in the host crystal form metastable energystate, at which electrons or positive holes excited from the groundstate by means of radiation are trapped. Then, when the crystal isheated to a sufficiently high temperature, electrons or positive holestrapped in the metastable state are released by means of thermalactivity and brought back to the ground state, emitting luminescence inthe visible or nearvisible wavelength range.

Most phosphors show thermoluminescence at room temperature or even atlower temperature due to relatively shallow metastable energy statethereof, and therefore gradually emit thermoluminescence at roomtemperature or lower to lose the energy accumulated therein afterexposure to radiation. Namely therefor materials show marked fading, andconsequently it is impossible to determine exactly the cumulative doseof radiation within a determined period. For use in thermoluminescentdosimeter, the phosphor is required to be provided with a trapping levelor metastable state of a proper energetic depth, but such property isonly found in very limited number of phosphors.

The conventional radiothermoluminescent materials employed in thedosimetry of radiation such as LiF, Li B O :Mn, CaSO :Mn, CaF zMn, etc.,are associated with various drawbacks such as low sensitivity, narrowdosimetry range, high energy dependency, large fading or cumbersomehandling requirements, etc., and many developmental efforts have beenmade in order to conquer such drawbacks.

In the course of investigation along the principle indicated above, thepresent inventors found that a complex oxide phosphor consisting ofmagnesium oxidesilicon dioxide and containing a trace amount of terbiumor cerium as activator shows a strong thermoluminescence with glow peakat Ca. C under the excitation by electron beam or various radiations andtherefore can be utilized as a highly sensitive phosphor forthermoluminescent dosimeter. I

The radiothermoluminescence dosimeters according to the presentinvention are composed of materials which can be expressed by thegeneral formula:

wherein M stands for an effective activating element,

namely at least one of terbium and cerium: x stands for the number ofmoles of silicon dioxide to be used at the preparation of phosphor withrespect to 1 mole of magnesium oxide; and y represents the number ofgram atoms of activator M with respect to 1 mole of magnesium oxide. Thethermoluminescence output, namely light intensity or light sum of thephosphor is strongly influenced by the values of these x and y.

The following is a brief description of the drawings:

FIG, 1 illustrates the relationshio between the ratio of SiO /MgOdefined as x and the intensity of thermoluminescence obtained. Itillustrates the influence of variations in the composition of hostmaterial in a terbium activated magnesium silcate phosphor.

FIG. 2 shows the relationship between the concentration of terbium andthe intensity of thermoluminescence obtained with a phosphor of MgO.0.3SiO

FIG. 3 shows the relationship between the heating temperature and theintensity of thermoluminescence, i.e., glow curve, after irradiation ofX-ray on silicon dioxide-magnesium oxide activated with terbium.

FIG. 4(A) and FIG. 4(8) show the relationship between the exposure doseand the intensity of thermoluminescence when X-ray or gamma ray of 60 isirradiated on silicon dioxide-magnesium oxide activated with terbium.

FIG. 5 illustrates a simple thermoluminescent dosimetry method with aglow curve used to determine the dose of radiation stored.

FIG. 1 showing the relationship between the ratio of SiO /MgO or x andthe intensity of thermoluminescence, represents the influence ofcomposition of host material in terbium activated magnesium silicatephosphor as an example of the radiothermoluminescent materials accordingto the present invention. For the purpose of thermoluminescencedosimetry the intensity of thermoluminescence is preferred to be asstrong as possible, and it is readily observed from FIG. 1 that thevalue of x should be maintained within a range from 0.03 to 3.0.,preferably from 0.2 to 1.0.

Although the value of y is kept constant at 10 in the case ofabove-mentioned figure, the desirable range of cussed above, theintensity of thermoluminescence is preferred to be as strong as possiblefor the purpose of thermoluminescence dosimetry, and from this figure itis readily understandable that the value of y should be maintainedwithin a range of 10 to 3 X preferably 10 to 10. Although the abovediscussion is made on the value of x of 0.3, the desirable range for ymentioned above remains fundamentally unchanged so long as the value ofx is present within a range of 0.03 to 3.0. Further these ranges arefundamentally alien from the firing conditions of phosphor such asheating temperature, heating period, surrounding atmosphere, etc.

The material for the radiothermolumenescence dosimeter according to thisinvention can be prepared by using magnesium oxide or magnesium compoundeasily convertible to said oxide upon heating such as magnesiumcarbonate, magnesium hydroxide, magnesium suifate, etc., and silicondioxide or silicon compound easily convertible to silicon dioxide uponheating as the host material of radiothermoluminescent material, mixingsufficiently at least a member of terbium oxide, terbium compound easilyconvertible thereto upon heating, cerium oxide and cerium compoundeasily convertible theretoupon heating as the activator with said hostmaterial, and heating thus obtained mixture under air atmosphere in anelectric furnace followed by rapid cooling'and crushing if necessary.Said mixing can be carried out either by dry process on a ball mill orroll mill or by wet process in which said components are made into pasteby means of water or ethyl alcohol, or said components arecoprecipitated by means for example of hydroxides. Said heating is.generally carried out within a' temperature range of l,300 to 2,000C.The heating period generally ranges from 0.5 to 20 hours, depending onthe size of crucible used, charging amount in the crucible, etc.Particularly desirable result can be obtained by effecting the heatingwithin a temperature range of 1,500 to l,800C for 2 to 10 hours. it isalso possible to heat the obtained material again in an inert gasatmosphere such as argon or nitrogen inorder to enhance the intensity ofthermoluminescence by a few tens percent.

The material thus prepared is made into thermoluminescence dosimeters bymeans of sealing said material in a glass tube together with inert gas,or of solidifying said material, for example by sintering said material,by compressing said material with a small amount of tabletting agentsuch as potassuum bromide to form a tablet or'by embedding said materialin a thermoresist'ant resin such as fluorine resin or silicon resin. Forthis purpose, any other known mean or method for formingthermoiuminescent dosimeters is naturally applicable so long as theradiothermoluminescent material constitutes the essential component ofthe dosimeter.

FIG. 3 shows the relationship between the heating temperature and theintensity of thermoluminecence, i.e., glow curve, after irradiation ofx-ray on a radiothermoluminescence dosimeter composed of a complex oxideradiothermoluminescent material consisting of silicon dioxide-magnesiumoxide activated with terbium as an example of radiothermoluminescencedosimeters 1 according to the present invention. This glow curve ischaracterized by the narrow distribution thereof around a single peak atC. which is particularly favorable for use in radiation dosimetry. Forthe purpose of radiation dosimetry utilized is the main peak at ca.190C. Further, the smaller peak represented by broken line in thedrawing may appear by the irradiation of light on the sample prior tothe measurement of thermoluminescence, but disappears completely whenthe sample is kept completely away from light. FIG. 4 (A) and FIG. 4(B)show the relationship between the exposure dose and the intensity ofthermoluminescence when X-ray or gamma ray of 60 is irradiated on aradiothermoluminescence dosimeter composed of a complex oxideradiothermoluminescent material consisting of silicon dioxide-magnesiumoxide activated with terbium as an example of radiothermoluminescentmaterial according to the present invention.

As is clarified in the foregoing explanation, the the thermoluminescencedosimeters according to the present invention linearly responds to theextremely wide variation of exposure ranging from 10 to 5 X l0R(Roentgen), and consequently said dosimeter allows precise quantitativemeasurement of the dose within the range mentioned above. Furthermore,said dosimeter qualitatively permits the dosimetry from several tens Rto 10 R, and thus can be concluded to be extremely suitable for fordetermining the cumulative dose of various radiations such as X-ray,gamms ray, etc.

The characteristics explained in FIG. 3 and FIG. 4 remain fundamentallyunchanged when the ratio SiO MgO or x is present within a range of 0.03to 3.0 and when the concentration of terbium or y is present within arange of 10' to 3 X 10 The radiothermoluminescence dosimeter accordingto the present invention of which characteristics have thus far beendisclosed has various extremely useful advantages when applied todosimetry of cumulative dose of various radiation such as X-ray gammaray, etc. Firstly to be noted is the advantage resulting from theproperty of glow curve already shown in FIG. 3. The presence of peak inthe glow curve at ca. 190C significantly decreases the fading ofintensity of thermoluminescence after exposure to radiation, andtherefore enables the precise control over the dose of exposed radiationfor prolonged period and also the centralized measurement and control ofexposure dose even at distant place. For example, the fading after 60days at normal termperature is only less than 3 percent. Besides the notexcessively high peak temperature prevents the use of very hightemperature, deterioration of precision dueto thermal radiation and theuse of complicated heating device. Furthermore the glow curve providedwith single peak and narrow distribution without accompanying sub-peaksallows precise and accurate measurement by simple heating operationwithout requiring any preliminary thermal treatment, since any sub-peak,if present in the glow curve at a lower temperature region than the mainpeak, will change the dimension thereof with the lapse of time after theexposure to the radiation, preventing accurate measurement, and acomplicated thermal treatment should be applied to the sample prior tothe measurement in order to remove the effect of such sub-peaks.Further, a wide distribution of glow curve indicates, through notclearly observable as separate sub-peak, the presence of certain factorin the lower temperature region causing the time-dependent fading ofthermoluminescence. Furthermore a wide distribution of glow curverequires heating the sample to a temperature considerably higher thanthe temperature at which the main peak is formed, and thereforeaccompanies the elevated influence of thermal radiation to lightdetector from the heater and surroundings, thus limiting considerablythe accuracy and range of measurement. In such case, even if the heatingis interrupted midways without reaching the summit of glow curve inorder to decreasesuch undesirably effect of said thermal radiation,'theretentive portion maintained in the sample will give rise to a largeerror in the case of repeated use of sample.

The second advantage lies in the fact that the high thermoluminescentoutput and the linear response over a wide range of dose as shown inFIG. 4 not only allows precise measurement of low dose but also enablesto apply a single sample for various purposes without preparingclassified dosimeters. This point will be further clarified in thefollowing.

At the measurement of dose as low as several mR or even lower, theeffect of thermal radiation from the heater and surroundings as well asof other noises will deteriorate signal-to-noise ratio at heating stepand will make it impossible to obtain satisfactory accuracy unless thephosphor used is provided with a particularly highthermoluminescenceintensity. At low dose range, even a very weak thermalradiation will become a problem and it is nearly impossible to preventcompletely the effect thereof by means of ordinary technical means. Alsosuch complete prevention of effect of thermal radiation, if possible,will require very complicated expensive mechanism and will therefore behardly applicable for practical measuring equipment.

On the other hand, the radiothermoluminescence dosimeter according tothis invention, owing to very high thermoluminescent output of thematerial enables, the measurement of low radiation dose with highaccuracy without requiring any additional mechanism or devices forpreventing the effect of thermal radiation but'by means ofvery simpleheating system such as placing the sample on a heating plate. Forexample, the thermoluminescent output of radiothermoluminescencedosimeter according to this invention obtainable in example 2 describedlater amounts to 100 times of that of wellknown radiothermoluminescencedosimeter of LiF under excitation with gamma ray of 60 With suchradiothermoluminescent material it is possible to realize athermoluminescent dosimeter capable of determining extremely weaknatural radioactivity, as weak as 0.01 mR, with a high precision.

in addition to the astonishing capability for measuring such lowradiation dose, the this invention is capable of providing an extremelysmall detecting element. For example the measurement of low dosementioned above can be realized with phosphor sealed in a small glasstube with extrenal diameter 1.0 mm and length mm. The measurement ofextremely low dose with such miniaturized detector is far beyond theconventional concept of dosimeter and provides powerful means formeasuring local distribution of radiation dose. F urthermore theextremely wide linear response range means applicability of a singleelement from a low dose to a very high dose with sufficient accuracy.As'already explained the dosimeter according to this invention iscapable of measuring the dose as high as 10 R, which itself is hardlyrealizable in conventional radiothermoluminescent material. LiF, forexample, loses linear response thereof against exposure dose at severalhundred R or 1,000 R and begins to show so-called superlinearity, losingthe accuracy. Further the fact that a detecting element for highradiation dose is also applicable for low dose as explained above hasnever been achieved in prior dosimeters such as ionization chamber norin prior radiothermoluminescence dosimeters and enables to use a singledetector forevery purpose in every field.

Thus, according to this invention provided is a simple thermoluminescentdosimetry methods as disclosed in the following, with reference to FIG.5. It is also one of the objects of this invention to provide suchsimple and highly reliable dosimetry method. The theromoluminescenedosimeter according to this invention, which is indicated by D in thedrawing, is exposed to unknown amount of radiation such as X-ray,gammaray, etc., to store the exposed energy in said dosimeter, and thestored energy, or exposed dose of radiation is determined by the glowcurve of thermoluminescence generated upon heating. Thus, by calibratingthe linear relationship between the the dose and thermoluminescenceintensity or the area under the glow curve in advance, it is possible todetermine, from the output of dosimeter, the exposure dose directly inthe unit proper for the radiation, for example most commonly in Roentgenunit.

Radiothermoluminescent material showing glow peak at ca. C can also beproduced by employing thalium, indium, bismuth or tin as activatorinstead of terbium or cerium mentioned above, such material generallyshows weaker thermoluminescent intensity compared with the caseactivated with terbium or cerium or accompanies sub-peaks in addition tothe main peak located at ca. 190C, and therefore is inferior as thephosphor for thermoluminescent dosimeter.

Though the description herein only refers to the use of Tb O as terbiumoxide, the compound expressed by Tb O naturally gives completely sameresult, and the amount of terbium in this invention is only specified interms of the number of gram-atom of elementary terbium with respect to 1mole of magnesium oxide.

This invention will be further clarified by the following examples.

EXAMPLE 1 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide(SiO )0.3 molesTerbium oxide "rb on 0.0005 moles were mixed sufficiently in a ball millor roll mill and then heated at 1,700 C for 2 hours in air in an aluminaor quartz crucible to obtain radiothermoluminescent material showingthermoluminescence with glow peak at ca. 190C as shown in FIG. 3 underexcitation by means of X-ray or various radiations.

EXAMPLE 2 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows: I

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide(SiO 0.5 molesTerbium oxide (Tb O 0.0007 moles were mixed sufficiently in a ball millor roll mill and then heated at l,600C for hours in air in athermoresistant container such as alumina or quartz crucible to obtainradiothermoluminescent material showing thermoluminescencewith glow peakat ca. l90C as shown in FIG. 3 under excitation by means of X-ray orvarious radiations.

EXAMPLE 3 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium carbonate (MgCO 1 mole Anhydrous silicon dioxide (SiO 0.3moles Terbium oxide (Tb O 0.0005 moles Mangesium carbonate was heated at1,000C for 2 hours in an alumina or quartz crucible, then mixedsufficientlywith the above other materials in a ball mill or roll milland again heated at 1,600C for 2 hours in air in an alumina or quartzcrucible to obtain radiothermoluminescent material showingthermoluminescence with glow peak at ca. 190C as shown in FIG. l underexcitation with X-ray or various radiations.

EXAMPLE 4 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium sulfate M so,.7H o 1 mol Anhydrous silicon dioxide (SiO 0.3moles Terbium oxide (Tb O 0.001 moles Magnesium sulfate was heated at700C for 1 hour in air in an alumina or quartz crucible to obtainanhydrous magnesium sulfate, which is then mixed sufficiently with theabove other materials on a ball mill or roll milland again heated atl,600C for'3 hours in an alumina or quartz crucible to obtainradiothermoluminescent material showing thermoluminescence with glowpeak at ca. 190C as shown in FIG. 3 under excitation with X-ray or otherradiations.

' EXAMPLE 5 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which as prepared as follows:

Magnesium hydroxide (Mg(()l-l) 1 mole Anhydrous silicon dioxide (Slo 0.3moles Terbium oxide (Tb O 0.0007 moles were mixed sufficiently on a ballmill or roll mill and heated at l,800C for 2 hours in air in an aluminaor quartz crucible to obtain radiothermoluminescent material showingthermoluminescence with glow peak at ca. 190C under excitation withX-ray or various radiations.

EXAMPLE 6 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO) 0.5 molesTerbium oxide (Tb O 0.0005 moles were sufficiently mixed on a ball millor roll mill and the heated at l,800C for 3 hours hair in an alumina orquartz crucible to obtain radiothermoluminescent material showingthermoluminescence with glow peak at ca. C under excitation with X-rayor other radiations.

EXAMPLE 7 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Silicon dioxide (SiO .nH O) O;3 moles Magnesium oxide (MgO) 1 moleTerbium oxide (Tb O 0.0007 moles Silicon dioxide was heated at l,000C inair for 2 hours in an alumina or quartz crucible to obtain anhydroussilicon dioxide, which was then sufficiently mixed with the above othertwo materials on a ball mill or roll mill and heated at 1,700C in airfor 5 hours in an alumina or quartz crucible to obtainradiothermoluminescent material showing thermoluminescence with glowpeak at ca. 190C under excitation with X-ray or other variousradiations.

EXAMPLE 8 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO 0.3 molesTerbium nitrate (Tb(NO .6H O) 0.001 moles were mixed sufficiently on aball mill or roll mill, then heated at 1,600C in air for 2 hours in analumina or quartz crucible and further heated at l,000C for 1 hour in aninert gas atmosphere, for example, argon has stream of How rate of ll./min to obtain radiothermoluminescent material showingthermoluminescence, with glow peak at ca. 190C and with an intensity ca.15 percent hlgher than that obtainable by heating in air, underexcitation with X-ray or other radiations.

EXAMPLE 9 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO 0.2 molesTerbium oxide (Tb O 0.0005 moles were mixed sufficiently on a ball millor roll mill, and heated at l,500C in air for 5 hours in athermoresistant container such as alumina or quartz crucible. Themixture was further mixed and heated at 1,200C for 2 hours in an aluminaor quartz tube under an inert gas atmosphere, for example, nitrogen gasstream of flow rate of 2 l./min to obtain radiothermoluminescentmaterial showing thermoluminescence under excitation with X-ray or otherradiations, with glow peak at ca. 190C as shown in HO. 3 and with anintensity ca. 20 percent enhanced than that obtainable by heating inair.

EXAMPLE 10 A radiothermoluminescence dosimeter was com-- posed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO 0.2 molesTerbium oxide (Tb O 0.007 moles were mixed sufficiently on a ball millor roll mill and heated at 1,800C in air for 2 hours in an alumina orquarts crucible. The mixture was further mixed, and heated at 1,800C inair for 1 hour. This heating was cooled rapidly to obtainradiothermoluminescent material showing strong theromoluminescence withglow peak at ca. 190C under excitation with X-ray or various radiations.

EXAMPLE 1 l A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows: I

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO 0,3 molesCerium oxide (Ce O 0.0005 moles were mixed sufficiently on a ball millor roll mill, then heated at 1,500C in air for 2 hours in an alumina orquartz crucible and cooled rapidly to obtain radiothermoluminescentmaterial showing thermoluminescence with glow peak at ca. 190C underexcitation with X-ray or various radiations.

EXAMPLE 12 A radiothermoluminescence dosimeter was composed of theradiothermoluminescent material which was prepared as follows:

Magnesium oxide (MgO) 1 mole Anhydrous silicon dioxide (SiO 0.3 molesCerium nitrate (Ce(NO .6I-l O) 0.01 moles were mixed sufficiently on aball mill or roll mill and heated at 1,600C in air for 5 hours in athermoresistant container such as alumina or quartz crucible to obtainradiothermoluminescent material showing thermoluminescence with glowpeak at ca. 190C as shown in FIG. 3 under excitation with X-ray orvarious radiations.

What we claim is:

1. A radiation dosimetrymethod which comprises exposing a dosimeteressentially consisting of complex oxide radiothermoluminescent materialcomposed of magnesium oxide-silicon oxide and activated with terbiumand/or cerium to a radiation in unknown dose and then measuringthermoluminescence emitted from said dosimeter upon heating therebyreading out the dose of said radiation.

2. A radiation dosimtery method according to claim 1 wherein a dosimeteressentially consisting of complex oxide radiothermoluminescent materialcomposed of magnesium oxide-silicon oxide with molar ratio of 1 0.03 tol 3.0 and activated withterbium and/or cerium in an amount of to 3 X 10'gram-atom with respect to 1 mole of magnesium oxde is used.

3. A radiation dosimetry method according to claim 1 wherein a dosimeteressentially consisting of complex oxide radiothermoluminescent materialcomposed of magnesium oxide-silicon oxide with molar ratio of 1 0.2 to 11.0 and activated with terbium and/or cerium in an amount of 10 to 10gram-atom with respect to 1 mole of magnesium oxide is used.

4. A radiation dosimetry method according to claim 1 wherein a dosimeteressentially consisting of complex oxide radiothermoluminescent materialcomposed of magnesium oxide-silicon oxide with molar ratio of 1 z 0.3 tol 0.5 and activated with terbium in an amount of 10' to 10' gram-atomwith respect to 1 mole of magnesium oxide is used.

5. A radiation dosimetry method according to claim 1, wherein adosimeter essentially consisting of complex oxide radiothermoluminescentmaterial composed of magnesium oxide-silicon oxide with molar ratio ofabout 0.3 and activated with terbium in an amount of 10 to 10 gram-atomwith respect to 1 mole of magnesium oxide is used.

6. A radiation dosimetry method according to claim 5, wherein saidterbium is in an amount of about 10 gram-atom with respect to 1 mole ofmagnesium oxide.

7. A radiation dosimetry method which comprises exposing a dosimeterconsisting essentially of complex oxide radiothermoluminescent material,composed of magnesium oxide-silicon oxide and activated with terbiumand/or cerium to irradiation in an unknown dose and subsequently,measuring the thermoluminescence emitted from said dosimeter uponheating, thereby reading out the dose of said radiation,

said radiothermoluminescent material being obtained by mixing:

1 magnesium oxide or a magnesium compound easily convertible theretoupon heating,

2. silicon oxide or a silicon compound easily convertible to siliconoxide upon heating, in an amount of from 0.2 to 1.0 moles of siliconoxide with respect to 1 mole of said magnesium oxide, and

3. terbium or compounds thereof in an amount of 10 to 10' gram-atom withrespect to 1 mole of said magnesium oxide, and

heating the thus obtained mixture at a temperature ranging from l,500 to1,800C., in the presence of air for 2 to 10 hours.

8. A radiation dosimetry method according to claim 7, wherein the molarratio of silicon oxide to magnesium oxide upon mixing is 0.3 to 0.5

9. A radiation dosimetry method according to claim 7, wherein the molarratio of silicon oxide to magnesium oxide is about 0.3 and thegram-atomic ratio of terbium to mangesium is about 10*.

10. A radiation dosimetry method according to claim 7, wherein aftersaid material is heated from l,500 to 1,800C in air for 2 to 10 hours itis then heated again at a temperature ranging from 1,000 to 1,500C for lto 5 hours in an argon or nitrogen atmosphere.

2. A radiation dosimetry method according to claim 1 wherein a dosimeteressentially consisting of complex oxide radiothermoluminescent materialcomposed of magnesium oxide-silicon oxide with molar ratio of 1 : 0.03to 1 : 3.0 and activated with terbium and/or cerium in an amount of 10 5to 3 X 10 2 gram-atom with respect to 1 mole of magnesium oxde is used.2. silicon oxide or a silicon compound easily convertible to siliconoxide upon heating, in an amount of from 0.2 to 1.0 moles of siliconoxide with respect to 1 mole of said magnesium oxide, and
 3. terbium orcompounds thereof in an amount of 10 3 to 10 2 gram-atom with respect to1 mole of said magnesium oxide, and heating the thus obtained mixture ata temperature ranging from 1,500* to 1,800*C., in the presence of airfor 2 to 10 hours.
 3. A radiation dosimetry method according to claim 1wherein a dosimeter essentially consisting of complex oxideradiothermoluminescent material composed of magnesium oxide-siliconoxide with molar ratio of 1 : 0.2 to 1 : 1.0 and activated with terbiumand/or cerium in an amount of 10 3 to 10 2 gram-atom with respect to 1mole of magnesium oxide is used.
 4. A radiation dosimetry methodaccording to claim 1 wherein a dosimeter essentially consisting ofcomplex oxide radiothermoluminescent material composed of magnesiumoxide-silicon oxide with molar ratio of 1 : 0.3 to 1 : 0.5 and activatedwith terbium in an amount of 10 3 to 10 2 gram-atom with respect to 1mole of magnesium oxide is used.
 5. A radiation dosimetry methodaccording to claim 1, wherein a dosimeter essentially consisting ofcomplex oxide radiothermoluminescent material composed of magnesiumoxide-silicon oxide with molar ratio of about 0.3 and activated withterbium in an amount of 10 3 to 10 2 gram-atom with respect to 1 mole ofmagnesium oxide is used.
 6. A radiation dosimetry method according toclaim 5, wherein said terbium is in an amount of about 10 3 gram-atomwith respect to 1 mole of magnesium oxide.
 7. A radiation dosimetrymethod which comprises exposing a dosimeter consisting essentially ofcomplex oxide radiothermoluminescent material, composed of magnesiumoxide-silicon oxide and activated with terbium and/or cerium toirradiation in an unknown dose and subsequently, measuring thethermoluminescence emitted from said dosimeter upon heating, therebyreading out the dose of said radiation, said radiothermoluminescentmaterial being obtained by mixing:
 8. A radiation dosimetry methodaccording to claim 7, wherein the molar ratio of silicon oxide tomagnesium oxide upon mixing is 0.3 to 0.5
 9. A radiation dosimetrymethod according to claim 7, wherein the molar ratio of silicon oxide tomagnesium oxide is about 0.3 and the gram-atomic ratio of terbium tomagnesium is about 10
 3. 10. A radiation dosimetry method according toclaim 7, wherein after said material is heated from 1,500* to 1,800*C inair for 2 to 10 hours it is then heated again at a temperature rangingfrom 1,000* to 1,500*C for 1 to 5 hours in an argon or nitrogenatmosphere.